So I think we can confidently skip the obvious thing about meleeing enemies. My question is which melee attacks count? Can I use my Arc Blade super to achieve this? How rapid is rapid? Are we talking consequtivly or just within 10 seconds or something?
No matter how hard I try, I've not been able to complete this. I've been trying it on the Moon against Hive Thralls.
If you have the first dlc the best location is the "Fist of Crota" mission on Earth — after the Sword bearer spawns you can pick up the sword and run to the next area where three packs of thralls attack — plenty enough for a quick melee chain.
Source: Just did it myself after trying several other farming spots.
As far as I have seen from youtube and such most people have no longer than 3 seconds between each consecutive kill. It can be achieved whilst in arc blade aswell.
As for time between kills? It's a rough estimate as only seen people achieve with kills in a 3 second window consecutively but to narrow down a hard factual number would be difficult I imagine unless Bungie suddenly announce it or some serious testing! :P
Rapid melee attacks also work on The Dark Beyond story mission. After the cutscene, the doors open, where the hive rush you in a decent size group. This has worked for the bounty for all of my characters, titan, warlock,and hunter